NASA astronauts Barry “Butch” Wilmore and Terry Virts have completed another EVA on Wednesday, the second of three spacewalks to begin the arduous process of reconfiguring the exterior of the International Space Station for the arrival of future commercial crew vehicles. The spacewalk also paid a visit to the end of the Station’s Canadarm2 for lubing tasks.
The EVAs are in support of the long-planned ISS reconfiguration from its current configuration, which was designed to support visiting Space Shuttles, to its new configuration optimised for future visiting commercial crew and cargo vehicles.
While cargo vehicles attach to the ISS using the process of berthing, whereby they are captured with the station’s robotic arm and positioned below a berthing port prior to being bolted into place, commercial crew vehicles will not use this method.
This is because the process of un-berthing takes a long time to complete, since cables and ducting between the visiting spacecraft and the ISS must first be manually disconnected, control boxes installed, hatches closed, and then the visiting spacecraft must be maneuverered away from the station with the robotic arm.
This means that berthing ports cannot support a rapid evacuation of crew from the ISS should it ever be necessary, which will be one of the primary roles of the commercial crew vehicles as they serve as “lifeboats” during their crew’s stay at the ISS.
Instead, crewed vehicles attach to the ISS via a process of docking, whereby the visiting spacecraft flies itself all the way into its docking port and attaches via a capture ring striking a corresponding attachment mechanism.
This process is how the Space Shuttles used to attach to the ISS during their now historic visits to the station.
However, since only one Space Shuttle ever visited the ISS at any one time, only one docking port was required – which was Pressurised Mating Adapter-2 (PMA-2) located at the forward end of Node 2.
There was another unused PMA on the ISS available for docking (PMA-3), however since it was not needed, it was placed in a stowage location on the Port side of Node 3, where it was unusable as a docking port due to clearance issues with other station hardware.
This Shuttle-oriented arrangement is still the configuration of the ISS to this day. For commercial crew vehicles however, it will need to change, since the new crew vehicles will require two separate docking ports.
This is because NASA want to have one primary and one back-up docking port in case one port should fail. Additionally, having two docking ports will open up the possibility of having two commercial crew vehicles visit the ISS simultaneously under the “taxi” model, although at this time it is only planned to have one vehicle at the ISS at any one time, under the “rent-a-car” model.
To this end, PMA-3 must be relocated from its currently unusable position on the Port side of Node 3, to the Zenith port of Node 2 at the very front end of the station, which is currently expected to occur around the middle of this year. Additionally, two adapters must then be installed onto both of the PMAs in order to convert their docking mechanism into one which will be used by the commercial crew vehicles.
The Space Shuttles used to dock to the PMAs using the Russian-designed Androgynous Peripheral Attachment System (APAS) hardware, which required a capture ring to impact the PMAs in order to provide initial capture. These impacts however are bad for the structural integrity of the ISS since, over time, they lead to micro-cracking in the module structures.
With the emphasis now on preserving the lifetime of the ISS beyond what was originally planned, a new docking system is needed which does not require a capture ring to impact the PMAs in order to provide capture. This new docking system is known as the Soft Impact Mating Attenuation Concept (SIMAC).
Thus, two International Docking Adapters (IDAs) need to be installed onto the end of both PMAs in order to convert their legacy APAS mechanism into a SIMAC mechanism. These two IDAs will be delivered to the ISS inside the Trunks of Dragon spacecraft later this year, and it is the power and data cabling for these IDAs is the primary purpose of the first two EVAs.
Relocating PMA-3 to the Zenith port of Node 2 creates an issue however, since Node 2 Zenith is currently used as the back-up berthing port for cargo vehicles. Thus, a new back-up port will need to be found, that does not have any clearance issues and is accessible for use as a cargo vehicle port. The primary cargo port will continue to be Node 2 Nadir as it is currently.
For this reason, the Permanent Multipurpose Module (PMM) must be relocated from its current home on Node 1 Nadir, to the Node 3 Forward port, also expected to occur around the middle of this year. This will free up Node 1 Nadir to serve as the cargo vehicle back-up port.
This move will also make it easier for two cargo vehicles to visit the ISS simultaneously, since presently a vehicle cannot be berthed directly to the current back-up port (Node 2 Zenith) due to SSRMS reach issues, instead requiring a complicated procedure of relocating the SSRMS, followed by relocating one of the cargo vehicles from the Node 2 Nadir to Zenith port prior to the second vehicle arriving.
This complex procedure means that presently it is easier to simply de-conflict cargo vehicle flights, however when the back-up cargo port moves to Node 1 Nadir, it will be possible to directly berth a vehicle to that port while a vehicle is present on the Node 2 Nadir primary port, thus enabling dual cargo vehicles to visit the station simultaneously.
Rounding out the ISS reconfiguration activities will be the arrival of the inflatable Bigelow Expandable Activity Module (BEAM) in a Dragon spacecraft later this year, which will be installed onto the Node 3 Aft port.
US EVA-29 procedures:
The opening EVA, designated US EVA-29, involved Barry Wilmore serving as EV-1 wearing the suit with the red stripes, and Terry Virts serving as EV-2 wearing the all-white suit, while ESA astronaut Samantha Cristoforetti served as the IV crewmember.
US EVA-29 was devoted entirely to routing and connecting cabling for the two IDAs, which is the most complicated cabling work ever attempted outside the US Segment of the station, with 10 different cables, which branch into 21 different legs, being involved in total.
Additionally, some of the connectors involved were never designed to be manipulated via EVA, and as such are not designed using the standard EVA-type connectors, since all US ISS modules were launched pre-wired as much as possible.
These type of connectors have been used before on EVAs on the Hubble Space Telescope, and a special tool is available from those missions to assist the spacewalkers should they need it.
While none of the tasks are really noteworthy in terms of being new or unique ops, and all of them being fairly mundane in terms of complexity, EVA cabling is notoriously fiddly work, hence why an entire spacewalk is being devoted to it.
The first order of business for Wilmore and Virts upon egressing the Quest Airlock was to translate out to the forward end of Node 2, taking with them two bags each containing the bundles of cables they will require, with each bundle being a different colour in order to visually aid the crewmembers.
The two spacewalkers then drove three bolts to remove a debris shield on the Port side of the Node 2 endcone, which exposed connectors beneath it (these are of the type not designed for EVA).
Four cables that used to provide power to the Space Shuttle and heating to PMA-2 were then disconnected, and four new IDA cables installed in their place. The debris shield was then re-installed.
A debris shield was then removed from the Starboard side of the Node 2 endcone, with one cable then disconnected and a new IDA cable reconnected in its place, followed by the debris shield being replaced.
One IDA cable was then be connected to a panel on the Destiny laboratory module, with one leg routed to and coiled near the Node 2 Zenith port in prep for the IDA-2 arrival, and the other leg routed to PMA-2 for use with IDA-1.
The rest of the EVA saw new cables being connected to PMA-2 itself and coiled near the forward end of PMA-2 for connection to IDA-1 later this year, with some legs also being routed and stowed for use with IDA-2 on PMA-3.
A get-ahead task was also conducted, involving work on extra cabling – mainly by staging it to save time ahead of the second EVA.
Following the retrieval of bags and clean up tasks, the crew translated back to Quest and ingressed the airlock to conclude the spacewalk after six hours and 41 minutes.
US EVA-30 procedures:
Wednesday’s US EVA-30 was very similar to EVA-29 in that it involved a lot of cabling work.
Wilmore was again designated EV-1 wearing the suit with the red stripes, with Virts as EV-2 wearing the all-white suit.
The first task involved the removal of a soft debris cover that has been protecting the PMA’s docking mechanism since 2013, in order to clear the way for the installation of IDA-1 later this year.
The cover was stowed in a bag and brought back inside.
Virts then disconnected a Visiting Vehicle power connector from the Destiny laboratory, in order to remove power from cables he worked with throughout the EVA, thus preventing so-called “hot mates” from occurring.
Following on from EVA-29, the duo then proceeded to mate connectors to both sides of PMA-2 and route them in prep for the IDA. With this completed, the power connector that was earlier disconnected was re-mated back at the Destiny lab. The PMA-2 worksite was then cleaned up and all bags returned to the airlock.
The next task for Virts was for him to ingress an Articulating Portable Foot Restraint (APFR) and install it on the Space Station Remove Manipulator System (SSRMS) arm, where he proceeded to apply lubrication to the Latching End Effector (LEE) of the SSRMS.
This involved extending and retracting the LEE latches a number of times while Virts used specially designed tools to apply grease to the area, although some of these tasks had to be done via feel only, since they were “blind ops” as the parts being lubricated were not visible.
Several “ball screws” were lubricated during this time.
Meanwhile, Wilmore translated to the Zenith toolbox on the Z1 Truss and retrieved a socket he required for his next task, which was the removal of a Non Propulsive Valve (NPV) from near to the Forward port of Node 3, since this could cause interference when the PMM is installed there.
A cover was installed in place of the NPV, following which a handrail was removed from the Node 3 Forward area, also to avoid clearance issues with the PMM.
Wilmore then proceeded to remove eight launch locks from four “petals” (two locks for each petal) from both the Node 3 Forward and Aft Common Berthing Mechanism (CBM) ports.
These petals cover the berthing collars of the ports and protect them from debris, and since the ports in question have not been used since Node 3 was launched in 2010, the petals had never been opened before now.
Once the launch locks were removed from the four petals on both the Forward and Aft Node 3 CBMs, ground control teams opened and closed the petals while Wilmore observed, to ensure they operated correctly.
Wilmore also opened two flaps on both the Node 3 Forward and Aft CBM ports, known as Centerline Berthing Camera System (CBCS) flaps. These flaps are basically window covers that protect the portholes in the CBM hatches from debris, however these flaps when closed prevent the CBCS camera views that are used to guide modules into place for installation purposes.
Thus, the Node 3 Forward and Aft CBCS flaps needed to be opened to allow for installation of the PMM and BEAM, respectively. This effectively means that the ISS has gained two new window views to the outside of the station.
Wilmore then stowed his socket back in the Z1 toolbox, whereupon he and Virts were to return to the airlock to conclude the EVA.
However, thanks to being over 75 minutes ahead of the timeline at one point, the ground team decided to give the duo some get-ahead tasks, including the attaching of wire ties to the S0 Truss in prep for US EVA-31.
Following the taking of closeout photos and final clean up tasks, victory was declared, allowing for the EVA to end at 18:34 UTC. The third and final EVA of these series will take place on Sunday.
However, this EVA was in doubt after water was noted in Virts’ EMU helmet during egress from the Quest airlock. The water was noticed by Virts during the end of the EVA, when the repress was at 5 psi.
It was only a small amount of water – classed as cold and also in the Helmet Absorption Pad (HAP) – but crew safety will result in this “leak” being investigated.
The amount of water – 15 ml – wasn’t close to that seen during the emergency of Luca Parmitano’s aborted EVA-23 in 2013.
Photos ere taken during the removal of the EMU suit equipment. The investigation concluded no issues for proceeding forward with Sunday’s spacewalk.
(Images: via L2 Special Sections and NASA).
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